Large woodland Nordmarka is the largest and most central of the Oslo woods, located North of the city.
Here you'll find several 'stuer'; small cabins that are often open during weekends to serve hikers snacks and refreshments. The more famous cabins are Kikutstua, Skjennungstua and the historic Frognerseteren (another afar.com highlight).
The woods are dotted with waters to dip your toes in and hills to scale, and there's even a tucked away hut used by the resistance during WWII that you can have a look at.